Bayonet: A blade which is attached to the end of a firearm for close quarter fighting; used by all nations.
Billjim: Slang for Australian soldiers, used by Australians.
Billy: Australian nickname for a cooking-pot or can.
Bonzer: Anzac slang for good.
Bullocky: Anzac slang for bad language.
Chats: Australian nickname for lice.
Digger: ANZAC soldier.
Dinkum: Australian colloquialism for 'good'.
Duck-Board: Slatted wooden planking used for flooring trenches or muddy ground.
Fire-Step: A step built into the trench wall for soldiers to fire or observe. The floor of the trench was lower so that the soldiers could walk upright without exposing their heads.
Kiwi: Term used to describe a New Zealander.
No Man's Land: The territory between the two front lines.
Over the Top: To leave the front trench and attack the enemy.
Trench Foot: Infection of the foot which could become gangrenous, caused by exposure to wet and cold.
Here are just a few words that you may be able to use in your letters.
If you want to check out some more there is a full A-Z glossary here >
http://europeanhistory.about.com/od/worldwar1/a/ww1glossb.htm
ANZAC: Australian and New Zealand Army Corps
Bayonet: A blade which is attached to the end of a firearm for close quarter fighting; used by all nations.
Billjim: Slang for Australian soldiers, used by Australians.
Billy: Australian nickname for a cooking-pot or can.
Bonzer: Anzac slang for good.
Bullocky: Anzac slang for bad language.
Chats: Australian nickname for lice.
Digger: ANZAC soldier.
Dinkum: Australian colloquialism for 'good'.
Duck-Board: Slatted wooden planking used for flooring trenches or muddy ground.
Fire-Step: A step built into the trench wall for soldiers to fire or observe. The floor of the trench was lower so that the soldiers could walk upright without exposing their heads.
Kiwi: Term used to describe a New Zealander.
No Man's Land: The territory between the two front lines.
Over the Top: To leave the front trench and attack the enemy.
Trench Foot: Infection of the foot which could become gangrenous, caused by exposure to wet and cold.
Tucker: Food.